As I've discussed, the Habitable Zone of a Sun-like star is calculated from the square root of the star's luminosity in terms of the Sun, multiplied by a constant for the inner and outer limits:

So, for the Sun, where L = 1, the optimistic (widest possible) Habitable Zone is calculated to extend from 0.75 AU to 1.77 AU, and the nucleal (optimal) Habitable Zone orbit at 1.00 AU.

The semi-major axes of the inner planets of the Solar System are:

Clearly, Mercury is well inside even the optimistic inner limit of the Sun's Habitable Zone (by 0.363 AU), while Venus is just barely too close (by 0.027 AU), and Mars is well within the optimistic outer limit (by 0.246 AU).

However, the Sun's luminosity has varied over its lifetime; shortly after its formation, its luminosity was about 70% of its current value, resulting in an average increase of about 6.7% per billion years. Thus, when the Sun was 1 billion years old, its luminosity was about 76.7% of its current value. Doing the calculations for the nucleal orbit and optimistic Habitable Zone based on this luminosity:

​... shows us that at that time, Venus was within the Habitable Zone of that time (by 0.066 AU), and Mars was still inside the outer limit (by 0.026 AU).

Thus, while Venus is demonstrably uninhabitable now, it is possible (perhaps likely) that Venus was far more habitable 3 billion years ago, especially if its rotational period were more-or-less what it is now, as is discussed in a Scientific American article from August 2016 titled Hellish Venus Might Have Been Habitable for Billions of Years [1].

The Sun's Future Habitable Zone

Similarly, the Sun's luminosity will increase as it ages, pushing the Habitable Zone limits outward from where they are now. By around 6.5 billion years from now—when the Sun is about 11 billion years of age and nearing the end of its Main Sequence lifetime—the Sun's luminosity will have increased to 2.2 times its current value, yielding the following figures:

So, at that time, Venus may very well be mostly molten, Earth will be desiccated, and Mars well within the nucleal orbit at that time (by 1.101 AU).

​Shortly thereafter, the Sun will enter its red giant phase and its luminosity will shoot up dramatically, as will its diameter, utterly engulfing Mercury and rendering Venus and Earth into hellish, lifeless balls of mostly molten rock. [2]

Stars Unlike the Sun

It is also important to remember that although the Habitable Zone limits are calculated using the luminosity of the star, they are measured in distance from its center of mass. Thus, a giant star like Aldebaran, with a luminosity of 518 times that of the Sun, has a nucleal habitable zone orbit of:

... which is slightly less (7.35 AU) that Uranus' orbit in the Solar System.

Aldebaran's radius is 44.2 times that of the Sun, or 695,700 ⨉ 44.2 = 30,749,940 km. One astronomical unit is 150,000,000 km, which means that Aldebaran's radius measures:

Thus, were Aldebaran to replace the Sun in the Solar System, its surface would fall only about0.387 - 0.205 = 0.182 AU inside Mercury's orbit.

The calculated nucleal orbit is 22.76 AU. An Earth-mass planet at that orbit would have an orbital period of:

... which is only about half (53.796%) of Uranus's orbital period.

The Innermost Stable Orbit for a star of 518 times the luminosity of the Sun is calculated by:

... which is 2.071 AU beyond the surface of the star.

Conclusion

So, we see that not only are habitable zones dependent on the radius and luminosity of the star, but also on the age of the star (especially for Main Sequence stars), because the luminosity changes during the normal lifetime, and the luminosity and the radius change in the later periods of the star's total lifetime.

Planets which were habitable in a star's youth may be uninhabitable in its later life (or destroyed altogether), while planets uninhabitable when the star is young may become habitable (for a time, at least) in its waning years.